Chelsea has been beaten 2-1 by Swiss football club Basel in their opening group match of the European Champions League at Stamford Bridge.
It is Chelsea's first home loss in the competition in 10 years.
Arsenal recovered from a slow start to beat Olympique Marseille 2-1 in France.
Lionel Messi moved into second place on the all-time list of Champions League goalscorers with his 27th career hat-trick as Barcelona beat Ajax 4-0 at the Nou Camp.
AC Milan beat Celtic 2-0 in Italy.